1. Field of the Invention
The invention relates in general to magnetic cores suitable for use in electrical inductive apparatus, such as transformers, and more specifically to new and improved methods of stress-relief annealing magnetic cores.
2. Description of the Prior Art
Magnetic cores constructed of amorphous ferromagnetic strip materials, such as the amorphous alloys available from Allied Corporation, must be stress-relief annealed in order to produce magnetic cores having the lowest core losses and the lowest exciting volt amperes. The effect of the stress-relief anneal is to partially relieve not only the residual stresses from the rapid quenching during production of the amorphous material, but also stresses introduced during core fabrication. A magnetic field applied during the stress-relief anneal process further reduces core losses and exciting volt amperes. However, the stress-relief anneal must be carried out at sufficiently low temperatures and for sufficiently short times to prevent degradation of the magnetic properties by crystallization of the amorphous ferromagnetic material. The resulting time-temperature window defined by the competing thermally-activated processes of stress relief and degradation is quite narrow, so the best magnetic properties are expected in small magnetic cores that can be annealed at essentially uniform temperature.
In large magnetic cores, thermal gradients during heating preclude truly uniform heating at reasonable rates, so that some parts of the magnetic core are expected to be at the stress-relief annealing temperature longer than other parts. The variation in heating rates is made greater by the anisotrophy of thermal conductivity in magnetic cores constructed of amorphous sheet or strip material. Because of the thermal barrier represented by the many gaps between the thin, nominally 1 to 11/2 mils, not-perfectly-smooth amorphous sheet layers, the thermal conductivity perpendicular to the plane of the sheet or strip material is very low. As a result, the exposed major sheet surfaces of the magnetic cores and adjacent lamination layers heat up very rapidly, because the heat is not rapidly conducted into the interior of the magnetic core. Although the thermal conductivity of amorphous material is not as high parallel to the sheet plane as that of regular grain oriented electrical steel, such as M-4, it is high enough that surface heat transfer from the furnace atmosphere to the magnetic core appears to be the process controlling the core heating rate, at least under presently used furnace conditions. Consequently, the temperature variation along the sheet plane is not as high as it is perpendicular to the sheet plane, and the layers of the magnetic core experience a different time-temperature history as the heat diffuses inwardly, yielding a nonuniform product from a magnetic viewpoint.